This invention relates generally to an oil level control system and particularly to an oil level control for multi-compressors used in a parallel refrigeration system with compressors that do not have oil pumps.
Historically, oil level control systems have been used with refrigeration compressors provided with internal oil pumps. These pumps receive their power from the compressor crankshaft and have an external pressure tap which is used to determine the oil pressure in the compressor. If the pressure is low, a pressure sensitive switch cuts off power to the compressor before damage can occur.
A recent development in the refrigeration industry for multi-compressor racks is the introduction of hermetic compressors using scroll technology. Scroll compressors do not use oil pumps with external taps for lubrication and therefore pressure sensitive switches do not protect the compressor from low oil lubrication damage. This invention provides that protection.
In a multi-compressor parallel refrigeration system, oil levels must be maintained in each compressor regardless of the oil circulation rate of each individual compressor. An oil control system is necessary to control low oil level conditions. In general, the primary components are an oil separator, an oil reservoir and an oil level sensor. Other secondary components may be optionally used.
These system components have been known for many years and are illustrated in Bulletins 110-10 and 110-20 of Sporlan Valve Company of Washington, Mo. These oil control systems respond to a low oil condition and will maintain the compressor oil level at a predetermined level.
Sporlan Valve Company Bulletin 110-10 discloses the use of an oil sensor that is a sealed steel ball providing a float. As the level of the oil rises and falls, the float to which a needle valve is attached rises and falls so that the valve opens and closes to maintain the correct oil level. Bulletin 110-20 also discloses the use of a float, in this case a pivotally mounted, machined float to which a magnet is attached. A hall-effect sensor is used to detect the position of a magnet on the float and therefore the oil level. As the level of oil changes, the angle of pivot changes and a signal is generated by the hall-effect sensor which opens and closes a solenoid valve to maintain the correct oil level. This system is described in patent application Ser. No. PCT/AU95/00354 (WO 95/35462) published Dec. 28, 1995. The disadvantage of this system is that ferrous particles tend to collect on the magnetized float, restricting the motion of the float.
Other developments in sensors are referred to generally as proximity detectors. One type of proximity detector is a conventional capacitive sensor. Capacitive sensors are useful for detecting certain materials based on their capacitance but they are generally unable to detect materials based on other properties such as resistance or inductance. Another type of proximity detector is an ultrasonic proximity detector which exploits reflections of sound waves off an object to detect the presence of the object. Ultrasonic detectors are generally unsuitable to detect material very close to the sensor. Still another type of proximity detector is an electromagnetic (EM) wave-based system. These detectors are similar to sonic detectors but use electromagnetic waves and suffer from the same deficiencies. Another type of proximity detector utilizes a tuned sensor element in which a monopole or dipole element is coupled to circuitry designed to be sensitive to the merit factor Q of the tuned circuit. Such sensors are typically tuned to detect high permittivity or high conductivity materials. U.S. Pat. No. 5,832,772 discloses a proximity detector of the latter type, and is incorporated herein by reference.
U.S. patent application Ser. No. 09/338,366 Macke, Sr. et al. filed Jun. 23, 1999 entitled Proximity Detector for Hard-to-Detect Material owned by BECS Technology, Inc. of St. Louis, Mo., discloses another type of proximity detector using a capacitative element in conjunction with particular circuitry for detecting the presence of a wide range of materials. This application is also incorporated herein by reference.
The present invention represents an improvement in these oil level systems in a manner not disclosed in the known prior art.